The invention relates to an X-ray tube comprising a cathode with an electron-emissive element and an anode with an anode target plate which are accomodated in an envelope having an exit window. An X-ray tube of this kind is known from U.S. Pat. No. 4,205,251.
For the study of elements having a comparatively low atomic number, for example lower than 30, by X-ray spectral analysis, known X-ray tubes are not ideally suitable because the X-rays generated therein contain an insufficient amount of long-wave X-rays. In order to generate comparatively soft and hence long-wave X-rays, use can be made of an anode material consisting of an element having a low atomic number. However, such an X-ray tube is not suitable for the study of elements having a high atomic number. Therefore, it is usually necessary to use several X-ray tubes for a complete analysis of an arbitrary specimen; this is annoying and time-consuming.